1. Field of the Invention
The present invention relates to a method and apparatus for measuring a Raman gain, a method and apparatus for controlling a Raman gain and a Raman amplifier, which are used in optical communications and the like. Particularly, the present invention relates to a method for measuring a Raman gain, which is capable of measuring a Raman gain at one end of an optical fiber transmission line, and an apparatus thereof, to a method and apparatus for controlling a Raman gain and to a Raman amplifier.
2. Description of Related Art
Along with exploitation of wavelength bands utilizing low-loss optical fibers and low-loss wavelength bands and with development of amplification technologies, prolongation of optical fiber transmissions has been advancing these days. To perform more efficient transmissions at lower costs, it is expected, in the future, to realize non-relay transmissions with low losses. For the non-relay transmission, conceived is application of an optical fiber amplifier, for example, an erbium-doped fiber optical amplifier (EDFA), which takes an optical fiber transmission line itself as an amplifying medium, or application of a broadband optical amplification technology.
In a communication system using such an optical fiber transmission line, progress has been made in development which aims at commercialization of a distributed Raman amplification (DRA) technology.
Raman amplification is a phenomenon in which signal light is amplified, the Raman amplification being operated in the following manner.
Specifically, when signal light and excitation light having a frequency higher than that of the signal light by about 13 THz are simultaneously inputted to an optical fiber made of silica glass, an energy of the excitation light is partially transferred to the signal light due to a stimulated Raman scattering phenomenon generated in the silica glass, whereby the signal light is amplified. A gain obtained by the Raman amplification is hereinafter referred to as a Raman gain. Actually, the Raman gain has wavelength dependency (Raman gain profile), as shown in FIG. 1, which has its peak at a wavelength having a frequency lower than that of the excitation light by 13 THz.
In addition, the distributed Raman amplification (DRA) is a form of obtaining a Raman amplification effect by taking an optical fiber transmission line itself as an amplifying medium. The DRA is realized by inputting excitation light to the optical fiber transmission line. In an optical fiber transmission system applied with the DRA, a transmittable distance can be extended because a propagation loss of the transmission line is compensated with the Raman amplification.
In an optical transmission system corresponding to the foregoing long-distance transmission, it is necessary to maintain a power of signal light which incurs a certain loss via the optical fiber transmission line at a desired level at a receiving side. Conventionally, an input level of the transmitted signal light has been measured at the receiving side, thereby adjusting a signal light power at a transmitting side or an amplification factor in a relay transmission line.
Description will be made hereinafter for a measuring method in a conventional optical transmission system. Here, for adjustment of the input level of the foregoing signal light, a gain efficiency of an optical fiber is particularly used. The gain efficiency is a parameter for each fiber, indicating how much gain can be obtained at a measuring point of the receiving side with respect to a power 1W of a transmitting light source. In other words, when excitation light of 1W is inputted to an amplifying medium, a Raman gain (dB) received by signal light propagating through the amplifying medium is called a Raman gain efficiency (dB/W). The Raman gain efficiency is different in each individual fiber. The reason for the difference in the Raman gain efficiency is that the Raman gain efficiency is influenced by such factors as a mode field diameter, an additive amount of GeO2, and absorption of water (OH), and that the above factors are different, respectively, for a type of the fiber, a manufacturer, time of manufacture and a lot. Furthermore, the Raman gain efficiency also fluctuates depending on a state at the scene, such as loss characteristics in a relay station. Thus, to control the amplification gain particularly in the distributed Raman amplification in the optical fiber transmission system using installed optical fiber transmission lines, it is necessary to measure the Raman gain efficiency.
In an attempt to measure the Raman gain efficiency, it was conventionally necessary to conduct an operation therefor by disposing measuring instruments, light sources and operators at both ends of the transmission line. In other words, it was necessary to operate a test light source disposed at one end of an installed fiber and a photodetector disposed at the other end thereof in conjunction with each other. In addition, because a manual operation was conducted by a maintenance worker going into the relay station to perform measurement, a procedure was required, that is, adjustment of a measurement timing, personnel deployment to the relay station, relocation of measuring instruments were required. As described above, because of difficulties in workability, there was a demand for means of measuring the gain efficiency only by an operation at one end of the transmission line.
Moreover, when, with respect to excitation wavelengths of plural types, Raman gain efficiencies of desired wavelengths are required to be obtained, respectively, different test lights having wavelengths of the same number as that of the types of the excitation wavelengths were necessary, thereby causing problems concerning costs, versatility and the like.